Field of the Invention
The present invention relates to an improved sacrificial isolation member for use with an isolation member seat to fluidically isolate a first portion of a well casing from a second portion of a well casing to expose a targeted geologic zone for hydraulic fracturing operations to enhance recovery and the rate of production of hydrocarbons from a well that penetrates the targeted geologic zone.
Background of the Related Art
Hydraulic fracturing is the fracturing of rock by a pressurized liquid. Some hydraulic fractures form naturally. Induced hydraulic fracturing or hydro-fracturing, commonly known as “fracking,” is a technique in which a fluid, typically water, is mixed with a proppant and chemicals to form a mixture that is injected at high pressure into a well to create small fractures in a hydrocarbon-bearing geologic formation along which the hydrocarbon fluids such as gas, oil or condensate may migrate to the well for production to the surface. Hydraulic pressure is removed from the well, then small grains of the proppant, for example, sand or aluminum oxide, hold the fractures open once the formation pressure achieves an equilibrium. The technique is commonly used in wells for shale gas, tight gas, tight oil, coal seam gas and hard rock wells. This well stimulation technique is generally only conducted once in the life of the well and greatly enhances fluid removal rates and well productivity.
A hydraulic fracture is formed by pumping fracturing fluid into a perforated section of the well at a rate sufficient to increase pressure downhole at the target zone (determined by the location of the well casing perforations) to exceed that of the fracture gradient (pressure gradient) of the rock. The fracture gradient is defined as the pressure increase per unit of the depth due to its density and it is usually measured in pounds per square inch per foot or bars per meter. The rock cracks and the fracture fluid continues further into the rock, extending the crack still further, and so on. Fractures are localized because pressure drop off with frictional loss attributed to the distance from the well. Operators typically try to maintain “fracture width,” or slow its decline, following treatment by introducing into the injected fluid a proppant—a material such as grains of sand, ceramic beads or other particulates that prevent the fractures from closing when the injection is stopped and the pressure of the fluid is removed. The propped fracture is permeable enough to allow the flow of formation fluids to the well. Formation fluids include gas, oil, salt water and fluids introduced to the formation during completion of the well during fracturing.
The location of one or more fractures along the length of the borehole is strictly controlled by various methods that create or seal off holes in the side of the well. A well may be fracked in stages by setting an isolation member seat, such as a ball seat or a plug seat, below the geologic formation to be fracked to isolate one or more lower geologic zones open to the well from the anticipated pressure to be later applied to a zone closer to the surface. An isolation member, such as, for example, a dart, a ball or a plug of a predetermined diameter and/or profile is introduced into the well to engage the corresponding isolation member seat. When the isolation member engages the isolation member seat installed in the bore of the well casing, the isolation member seats in the isolation member seat to form a seal that isolates the first portion of the casing below the seat from the hydraulic fracturing pressure to be imposed on a geologic formation zone in fluid communication with the second portion of the casing having perforations above the seat.
Hydraulic-fracturing equipment used in oil and natural gas fields usually consists of a slurry blender, one or more high-pressure, high-volume fracturing pumps (typically powerful triplex or quintuplex pumps) and a monitoring unit. Associated equipment includes fracturing tanks, one or more units for storage and handling of proppant, high-pressure treating iron, a chemical additive unit (used to accurately monitor chemical addition), low-pressure flexible hoses, and many gauges and meters for flow rate, fluid density, and treating pressure. Chemical additives are typically 0.5% percent of the total fluid volume. Fracturing equipment operates over a range of pressures and injection rates, and can reach up to 100 megapascals (15,000 psi) and 265 liters per second (9.4 cu. ft./sec or 100 barrels per min.).
A problem that can be encountered in a fracking operation involves the impairment to subsequent operations that can result from the presence of the isolation member engaged with the isolation member seat. After the fracking operation is concluded, the surface pressure is restored to a pressure at which the well will flow and produce formation fluids to the surface for recovery. An isolation member to be used for fracking and having a sufficiently low density can be floated or back-flowed from the well, but an isolation member having a low density may be deformed by the large pressure differential applied across the isolation member and the cooperating isolation member seat. Unwanted deformation of the isolation member may compromise the effectiveness of the fracturing operations. If the isolation member is of a material that is more dense so that it can not be floated or back-flowed from the well to the surface, then the isolation member may present an unwanted well obstruction that must be removed from the well to prevent impairment of subsequent well operations.
A workover operation can be implemented in which a drilling instrument is introduced into the well to drill out and to mechanically destroy the isolation member, but a workover operation requires that a workover rig be brought to the surface location of the well for downhole operations. The need for the rental, transportation, rigging up and use of a rig imposes substantial delays and substantial costs.
What is needed is an isolation member that can be used for fracking and that has a sufficient density and resistance to deformation so that it can be used in conjunction with a corresponding seat to reliably isolate geologic formation zones below the seat from anticipated large fracturing pressures applied to geologic formation zones above the seat and that does not impair subsequent well operations.